The invention relates to a lockwire retention system used to prevent axial movement of a turbine bucket dovetail in a corresponding dovetail slot in a turbine rotor wheel, and more specifically, to techniques for preventing circumferential rotation of the lockwire itself within an annular groove in the turbine rotor wheel.
In conventional turbine and/or turbine compressor components, buckets (or blades, or airfoils) are held in a rotor wheel by means of a slotted connection, e.g., a so-called fir tree or Christmas tree arrangement where an inwardly-tapered male connector portion at the radially inner end of the bucket is received in a complimentary female slot in the rotor wheel. Such connections are also generically referred to as “dovetail” connections, embracing various complimentary shapes which lock the buckets to the wheel in the radial and circumferential directions so as to accommodate the high centrifugal forces generated by rotation of the turbine rotor.
The fit between the blade dovetail and the dovetail slot is somewhat loose to allow for assembly and tolerances. Therefore, if the blades are not properly retained, the loose fit may allow the bucket or blade to move axially along the slot, leading to excessive wear or even collisions with neighboring components. The excessive wear can eventually fail the part, requiring the unit to be shut down until a repair is made. Bucket translation is particularly worrisome for cooled buckets. Small amounts of axial displacement can block the inflow of air into the part and lead to premature failure.
In accordance with one known practice, the buckets or blades are prevented from moving axially in the dovetail slots provided in the rotor wheel by a lockwire passing through an annular slot formed in the radially outer periphery of the wheel, bridging the dovetail slots, and passing through circumferentially-aligned slots in the dovetail portions of the respective buckets. The free ends of the wire are shaped so that they come together at an overlapped joint, thus allowing for minor changes in diameter as the airfoils move radially within the respective dovetail slots. The overlap joint is more to allow for thermal expansion/contraction of both the wire and rotor during transient periods. The lockwire is held in place by pins mounted in the turbine wheel, radially inwardly of the lockwire. It has been discovered that rotation of the lockwire within the annular slot in the rotor wheel (which occurs over time) can cause one end of the lockwire to engage a pin and bend downwardly (radially inwardly) below the pin and escape the annular slot. Without the lockwire, the airfoils are free to travel axially along the dovetail slots, creating the potential for excessive wear and interference as mentioned above. In addition, this is especially consequential in first stage buckets that rely on holes in the base of the bucket to provide internal cooling. When these holes are blocked due to axial movement of the bucket, the bucket can quickly oxidize along the leading edge.
There remains a need for a reliable technique for preventing rotation of the lockwire within its annular slot to thereby prevent escape of the lockwire from the rotor wheel.